Analysis of electrical signals produced by muscle activity has been found useful in a variety of monitoring and control applications. One of the most familiar is the electrocardiogram or "ECG" which utilizes a plurality of electrodes in contact with the surface of the skin at various locations on the human body to detect electrical waveforms indicative of cardiac activity. Analysis of such waveforms is useful in the diagnosis of many pathological conditions of the heart. Peter P. Tarjan, one of the present co-inventors, has also experimented with sensors and triangulation techniques for cardiac arrhythmia detection in canines as presented in 1987 at the World Symposium on cardiac Pacing in Jerusalem, Israel and at the North American Society for Pacing and Electro-Physiology in Boston, Mass.
It has also been known to detect the electrical activity of skeletal muscles, such as those in the area of the eyelids or eyebrows, in order to monitor an individual's state of consciousness or sleep. U.S. Pat. No. 4,359,724 to Zimmerman et al. relates to an eyelid movement detector which employs a bipolar electrode placed over each eyebrow of a person. The electrodes are coupled to a circuit which includes a timer which is reset in response to eyelid activity detected by the electrodes. If the timer fails to be reset within a predetermined time interval, it activates a warning device, such asian audible alarm, to warn the person that they are becoming drowsy. An alternative embodiment uses a third electrode positioned on the forehead between the other two electrodes to provide a reference signal level for improved noise immunity.
Efforts have also been made to control devices, such as the actuators of an artificial limb, according to electrical signals derived from electrodes responsive to skeletal muscle activity. Bipolar electrodes have been used by Stephen Jacobsen to detect myopotentials of the trapezius and deltoids for controlling the "Utah" artificial human arm and other prosthetic devices. However, biological control of prostheses or other non-biological systems has presented obstacles which have not been satisfactorily overcome.
Sequential input systems such as the oral pneumatic "sip and puff" devices used by some quadraplegics can be adapted for use with an arbitrarily large set of input commands but become progressively slower to use as the size of the command set increases. In systems wherein commands are determined by combinations of inputs, the number of control states which can be effected is limited by the available number of distinct biological input conditions which can be voluntarily produced by a subject and reliably recognized by the system to be controlled. While controls based on voice inputs have advanced remarkably in recent years and offer the ability to recognize large numbers of commands, voice-activated systems are not suitable for many applications such as those in aircraft cockpits or on the floor of a factory where a considerable amount of acoustic background noise may be present. Sensing of myopotentials can provide excellent immunity from such external environmental influences but has been restricted in practical utility. One problem has been the relatively small number of control input conditions discriminable using skin surface mounted electrode arrays of a given size and mutual spacing. This has been due to the tendency of known skin surface mounted electrodes to respond to the electrical activity of not only the muscle in the immediate vicinity of the electrode but also to that from relatively distant locations. For example, bipolar electrodes of the type used in Zimmerman et al. '724 are known to respond to electrical activity of muscle tissue located a considerable distance from that immediately underlying the electrode itself. This necessitates a relatively wide separation between electrodes in order to provide discriminable differences in their responses to different voluntary muscle activities. As a consequence the number of distinct "commands" discriminable by an electrode array of a given number of electrodes and mutual spacing is limited. In theory, this limitation can be overcome by using invasive electrodes.
Various types of implanted or invasive electrodes are known which respond to the local electrical activity of a nerve or muscle while not responding to the activity of closely adjacent nerves or muscles. Indeed, a microelectrode penetrating the surface of the body can be located to respond only to the activity of a single neuron or myofibril. However, the use of implanted or invasive electrodes is disfavored other than for short term laboratory or clinical use because they can cause discomfort and pose a risk of infection.